1. Field of the Invention
The present invention relates to a vacuum die-casting machine, more particularly relates to a vacuum die-casting machine which coats an inside of a mold cavity with a powder release agent, then evacuates the inside of the cavity and injects a molten metal (for example aluminum alloy).
2. Description of the Related Art
A conventional vacuum die-casting machine is described in Japanese Patent Publication (A) No. 9-277007 (see FIG. 3). This vacuum die-casting machine 900 clamps the die-casting dies, opens a shutoff valve 24 after clamping them, and, in that state, operates a not shown pressure reducing device (vacuum pump). Due to this, a mold cavity C is evacuated from an exhaust port 22 communicated with one side of the mold cavity C so as to reduce the pressure at the inside of the mold cavity C, while a powder release agent is fed from a melt passage 11 which is communicated with another side of the mold cavity to the inside of the mold cavity. The powder release agent is fed from a powder storage source T to a powder discharge passage 20, switch valve 19, switch flow path 13, and melt passage 11. When the powder release agent is fed into the cavity, the switch valve 19 switches the flow path whereby the powder discharge passage 20 and the switch flow path 13 are disconnected, the vacuum pump P and the switch flow path 13 are communicated, and air inside the cavity is sucked out and the inside of the cavity is held in a vacuum state. After that, the cylindrical valve 14 moves left whereby a molten metal holding chamber 8B inside the sleeve 8 and the cavity C are communicated. Further, the plunger 9 moves leftward to inject molten metal (aluminum alloy) which is filled in the molten metal holding chamber 8B to the cavity C and thereby fill the inside of the cavity C with the molten metal.
Further, inside a powder control valve guide hole G which is formed inside a sprue bushing 10 and a guide bushing 12, a powder control valve V is movably arranged. The powder control valve V is formed at its right end with a cylindrical valve part 14 which closely contacts the powder control valve guide hole G, is formed with a reduced size tube 15 from the cylindrical valve part 14 toward the left, is further formed with a guide tube 16 from the reduced size tube 15 toward the left, and is still further formed with an operating rod part 17 from the guide tube 16 toward the left. Between this reduced size tube 15 and guide hole G, there is a clearance volume 15A.
However, such a conventional vacuum die-casting machine suffers from the inconvenience that the powder release agent builds up in the clearance volume 15A which eventually causes the individual feed amounts of the power release agent into the mold cavity to become unstable. Furthermore, in the clearance volume 15A, the velocity of the air flow for sucking out the powder release agent is reduced. Furthermore, two vacuum pumps become necessary. The system becomes larger in size and the capital costs increase.
For more information on the related art, see the above-mentioned Japanese Patent Publication (A) No. 9-277007.